I for a while worked for RHM under the banner of Mothers Pride. We had state of the art BMC 2.2 diesel with a heavy body. Mine was automatic!!!!!! Top speed 45mph downhill with a Cornish gale behind it.
What on earth is this to do with heaters, well the body work was so well made when it rained or snowed, the wind use to blow it through one side and out the other. WITH the doors shut.
Heater NO the pipework was diverted through a 'wash up' so that we could have hot water to wash our hands. That was if you could feel them!!!!!!!!!!!!!!!!